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Parents who smoke outside their house are still exposing their children to the harmful effects of passive smoking, an Australian study suggests.

The study found that the levels of respirable suspended particles, including nicotine, were significantly higher in houses where smokers lived than in smoke-free homes - even if they only smoked outside.

Lead author of the study, Dr Krassi Rumchev of Curtin University of Technology, says the findings indicate that the level of passive smoking by children at home may be underestimated, as those whose parents smoked outside were exposed to levels of environmental tobacco smoke high enough to cause harm.

"[The] results demonstrate clearly that if parents want a smoke-free environment for children, they need to stop smoking."

She adds that children were more likely to have respiratory illnesses including asthma, coughs and colds than those in tobacco-free households.

Clinging on

The researchers urge doctors to advise parents to quit and make their homes completely smoke-free.

Rumchev says smoke-free public places have worked well and the next step is smoke-free homes for children.

Researchers measured nicotine and respirable particles over 24 hours in the living rooms of 92 Perth households with children aged between four and nine years old.

Although 39 houses (42%) had smokers, only 4% said smoking occurred inside.

Levels were low in homes without smokers and considerably higher in houses where smoking was reported.

"[The] findings are concerning, and it's a clear message that more education programs are required and we need to concentrate on making residential settings smoke-free," she says.

She says smokers appear to disperse pollutants into the home when returning inside.

"When people come inside they're still breathing out smoke and it contaminates the air. It's enough to do harm," Rumchev says.

"Nicotine attaches to the hair and body, and pollutants are dispersed into the air off clothes, because small particles can attach to clothes."

Coughs and wheezes

Half of the children in the study had lower respiratory symptoms, such as asthma, wheezing and shortness of breath, while 42% had upper respiratory symptoms, including coughing and runny nose.

Respiratory illness was more prevalent in households with smokers than smoke-free homes. Children exposed to higher air nicotine levels were three times more likely to have asthma or wheeze than those not exposed.

The study also found that those with coughs or runny noses were exposed to higher nicotine levels than children without these symptoms.

The researchers found that air-conditioning or opening windows did not sufficiently reduce environmental tobacco smoke.

They also say that particulate levels related to gas heating and cooking were accounted for in this study.

The study adds to previous research, which shows that environmental tobacco smoke impacts more in early childhood when the immune system and lungs are developing.

About 8% of childhood asthma and 13% of major lower respiratory illnesses in Australian children are due to passive smoking.